Organic light emitting device

ABSTRACT

It is an object of the present invention to provide an organic light emitting device having a long-life optical output. The organic light emitting device according to the present invention is provided with an emission layer including at least a host material, a light emitting material, and another material, wherein the another material has a smaller ionization potential than and almost the same hole mobility as or a greater hole mobility than an ionization potential and a hole mobility of a compound which forms an emission layer-side-interface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 12/545,995, filed Aug. 24, 2009, which in turn is a division of U.S.application Ser. No. 11/175,206, filed Jul. 7, 2005, issued as U.S. Pat.No. 7,604,873, which claims the benefit of Japanese Patent ApplicationNo. 2004-211231, filed Jul. 20, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting device using anorganic compound, and more specifically, to an organic light emittingdevice from which light is emitted by applying an electric field on athin film made of an organic compound.

2. Related Background Art

An organic light emitting device is a device that includes a thin filmmade of an emitting organic compound between an anode and a cathode,generates an exciton from the fluorescent organic compound by injectionof an electron and a hole from each electrode, and utilizes light to beradiated when the exciton returns to the ground state.

A study conducted by Eastman Kodak Company in 1987 (Appl. Phys. Lett.51, 913 (1987)) reported light emission in the order of 1,000 cd/m² atan applied voltage of about 10 V, from a device having aseparated-function type two-layer structure in which an anode is made ofITO and a cathode is made of a magnesium-silver alloy, an aluminumquinolinol complex is used as each of an electron-transporting materialand a light emitting material, and also a triphenylamine derivative isused as a hole-transporting material. Related patents include U.S. Pat.Nos. 4,539,507, 4,720,432, and 4,885,211.

In addition, luminance at spectra ranging from ultraviolet throughinfrared is allowed by changing the type of the fluorescent organiccompound. Recently, therefore, various compounds have been studiedextensively and described in, for example, U.S. Pat. Nos. 5,151,629,5,409,783 and 5,382,477, and Japanese Patent Application Laid-Open Nos.H2-247278, H3-255190, H5-202356, H9-202878 and H9-227576.

Furthermore, in addition to organic light emitting devices using lowmolecular weight materials as described above, an organic light emittingdevice using a conjugated polymer has been reported by a group fromCambridge University (Nature, 347, 539 (1990)). This report confirmedluminance from a monolayer by film formation with polyphenylene vinylene(PPV) in a coating system. Patents relating to an organic light emittingdevice using a conjugated polymer include U.S. Pat. Nos. 5,247,190,5,514,878, 5,672,678, and Japanese Patent Application Laid-Open Nos.H4-145192 and H5-247460.

Recent advances in technology concerning organic light emitting deviceshave been remarkable. The characteristics of such devices allow theformation of thin and lightweight light-emitting devices having highluminance at a low applied voltage, a variety of emission wavelengths,and high-speed response, suggesting that these devices could be put toextensive use. However, many problems still remain to be solved in termsof durability, such as luminescence degradation over time by prolongeduse.

In order to solve this problem, Japanese Patent Application Laid-OpenNo. 2002-43063 and U.S. Pat. No. 6,392,250 disclose a stacked-typeorganic light emitting device in which the emission layer is a mixedlayer consisting of hole-transporting material, electron-transportingmaterial and an emitter. This mixed layer suppresses the generation ofunstable cationic species (holes) of the electron-transporting material,whose purpose is to improve operational durability. However, such acombination of a hole-transporting material and electron-transportingmaterial has been often ineffective to improve the durability, andfurthermore, no consideration has been given to deterioration of theemitter. In addition, these patent documents basically relate to anemission layer, so that a combination thereof with other layers has beenoften found ineffective.

Japanese Patent Application Laid-Open No. 2003-151777 discloses anorganic light emitting organic device which employs a mixed layer of ahole-transporting material and an electron-transporting material, and iscapable of further adding a light emitting material. In this device,selecting a material different from the mixed layer for thehole-transporting region or electron-transporting region provides theadvantages of improving efficiency and stability due to blocking ofcharge leakage/exciton diffusion, increasing the degree of freedom inmaterial selection, and increasing economic efficiency. However, theabove advantages are merely the advantages of an ordinary stackedstructure, and are not just limited to a device using a mixed layer asan emission layer. In addition, no consideration is given todeterioration of the emitting material.

Japanese Patent Application Laid-Open No. 2000-106277 discloses anorganic light emitting organic device in which the emission layercontains a host material, a polycyclic aromatic hydrocarbon, and afluorescent dye, wherein the hole mobility of the polycyclic aromatichydrocarbon is greater than that of the host compound. This prior art isdirected to prolonging the life of a device by suppressing holeaccumulation in the host material of the emission layer by using a highhole-mobility polycyclic aromatic hydrocarbon. However, because thehighest occupied orbital of the fluorescent dye is at an energy levelequal to or higher than that of the polycyclic aromatic hydrocarbon,there is the problem that hole accumulation in the fluorescent dyeoccurs. In addition, this prior art basically relates to an emissionlayer, so that a combination thereof with other layers does not achievethe required effects in some cases.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an organic lightemitting device having a long-life optical output.

The present inventors have accomplished the present invention as aresult of their extensive studies into solving the above problem. Thatis, the organic light emitting device according to the present inventionincludes: a pair of electrodes, which consist of an anode and a cathode,and a plurality of organic layers interposed between the pair ofelectrodes, wherein the plurality of organic layers includes at least anemission layer and another organic layer which is in contact with ananode-side-interface of the emission layer, and wherein the emissionlayer includes at least:

(1) a host material;

(2) a light emitting material; and

(3) another material having a smaller ionization potential than andalmost the same hole mobility as or a greater hole mobility than anionization potential and a hole mobility of a compound which constitutesan emission layer-interface-side of the another organic layer.

Accordingly, the present invention can provide an organic light emittingdevice remarkably excellent in stability with elapse of time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an example of the organic lightemitting device in accordance with the present invention;

FIG. 2 is a cross-sectional view showing another example of the organiclight emitting device in accordance with the present invention;

FIG. 3 is a cross-sectional view showing still another example of theorganic light emitting device in accordance with the present invention;

FIG. 4 is a cross-sectional view showing still another example of theorganic light emitting device in accordance with the present invention;

FIG. 5 is a cross-sectional view showing still another example of theorganic light emitting device in accordance with the present invention;

FIG. 6 is a diagram explaining the principle of the prolonged life inaccordance with the present invention;

FIG. 7 is an example of an energy diagram of an organic light emittingdevice in accordance with the present invention;

FIG. 8 is an energy diagram of the hole-transporting layer and theemission layer of the organic light emitting device according to Example1 of the present invention; and

FIG. 9 is an energy diagram of the hole-transporting layer and theemission layer of the organic light emitting device according to Example5 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail. FIGS. 1 to 5 areschematic diagrams which illustrate structural examples of the organiclight emitting device in accordance with the present invention.

First, the reference numerals in each of the drawings will be explained.

Reference numeral 1 denotes a substrate, 2 an anode, 3 an emissionlayer, 4 a cathode, 5 a hole-transporting layer, 6 anelectron-transporting layer, 7 a hole injection layer, 8 ahole/exciton-blocking layer and 9 an electron/exciton-blocking layer.

FIG. 1 shows an organic light emitting device constructed by mountingthe anode 2, the hole-transporting layer 5, the electron-transportinglayer 6 and the cathode 4 on the substrate 1 in mentioned order. In thissituation, the light emitting region is in the electron-transportinglayer 6, wherein the electron-transporting layer 6 serves as theemission layer.

FIG. 2 shows an organic light emitting device constructed by mountingthe anode 2, the hole-transporting layer 5, the emission layer 3, theelectron-transporting layer 6, and the cathode 4 on the substrate 1 inmentioned order. In this case, the carrier-transportation function andthe luminescence function are separated, wherein compounds having eachof the characteristics of hole-transport, electron-transport andluminance can be used in a suitable combination. This greatly increasesthe degree of freedom of selecting materials, as well as enabling avariety of compounds which differ in their wavelength to be used, whichin turn allows the luminance hue to diversify. Further, by effectivelyconfining each carrier or exciton in the middle emission layer 3,luminescence efficiency can be increased.

FIG. 3 is different from FIG. 2 in that the hole injection layer 7 isinserted in the layer structure on the side of the anode 2. Theinsertion is effective in improving the close contact between the anode2 and the hole-transporting layer 5 or improving the hole-implantingability. Therefore, such a configuration of the device will beadvantageous in lowering the voltage of the device.

FIG. 4 is different from FIG. 2 in that a layer for preventing a hole orexciton from traveling toward the side of the cathode 4(hole/exciton-blocking layer 8) is inserted between the emission layer 3and the electron-transporting layer 6. Using a compound having a largeionization potential as the hole/exciton-blocking layer 8 efficientlyprovides the configuration of the device with improved luminescenceefficiency.

FIG. 5 is different from FIG. 2 in that a layer for preventing anelectron or exciton from traveling toward the side of the anode 2(electron/exciton-blocking layer 9) is inserted between the emissionlayer 3 and the hole-transporting layer 5. Using a compound having asmall electron affinity as the electron/exciton-blocking layer 9efficiently provides the configuration of the device with improvedluminescence efficiency.

A structure such as that described above often adopts a structure as theemission layer in which a luminescent dopant (light-emitting material)is doped into the host material. Doping increases the degree of freedomwith which the respective functions of charge mobility, recombination,and luminescence can be optimized in the emission layer, wherebyluminescence efficiency can be improved and a diversified luminance huecan be achieved.

The emission layer can also be formed from a plurality of layers eachhaving a different luminescent color, wherein the emissions from therespective layers are mixed for use.

The present invention relates, among the device structure examplesexplained above, to a structure having an emission layer and anotherorganic layer provided on the anode side of the emission layer. Thisanother organic layer denotes the hole-transporting layer in FIGS. 1 to4, and the electron/exciton-blocking layer in FIG. 5. While theprinciple of the present invention is explained using the structureshown in FIG. 2, the present invention is not limited thereto, and canachieve its effects in a similar manner even with a different structure.

As a result of extensive study on the causes of luminance degradationwhich occurs when an organic light emitting device is operated, thepresent inventors have ascertained that a major cause of degradation ishole accumulation in the hole-transporting region, and further, holeaccumulation in the emitting material contained in the emission layer,and have proposed a method to solve this problem.

The principle of prolonging life in accordance with the presentinvention will be explained using the energy diagram shown in FIG. 6. Inthe present invention, the emission layer is constituted from at leastthree kinds of material. These three kinds are a host material, a lightemitting material and another material that will be explained below.Compared with a compound which constitutes the region Y on the emissionlayer-interface-side of the hole-transporting material, this anothermaterial: (1) has a smaller ionization potential (i.e. a lower HOMOenergy level); and (2) has almost the same or greater hole mobility(i.e. the holes move more rapidly). This another material is preferablycontained at least in the region Z of the hole-transporting layer sideof the emission layer. “Almost the same hole mobility” can mean a holemobility ratio in the range of from 0.2 to 5 or more.

Based on the action of this another material, the holes implanted fromthe anode can be expected to be quickly implanted into the emissionlayer after being transported through the hole-transporting layer. As aresult, the hole accumulation in the hole-transporting layer ismoderated. When the another material of the present invention is notpresent, holes accumulate at the emission layer-side-interface of thehole-transporting layer, thereby becoming a factor in devicedeterioration.

To prevent hole accumulation in the emitting material contained in theemission layer, it is effective when the ionization potential of theabove another material is less than the ionization potential of theemitting material (i.e. the another material has a lower HOMO energylevel). This is because the holes implanted into the emission layerpreferentially flow through the energy-stable another material incomparison with the emitting material.

Thus, holes implanted into the emission layer recombine with electronsimplanted from the cathode to thereby generate an exciton, whereuponlight is emitted mainly from the emitting material. Althoughrecombination preferably takes place on the emitting material molecules(direct recombination luminescence), excitons that recombine on a hostmaterial molecule or another material molecule also move by their energyto the emitting material molecule, whereby the emitting materialultimately emits light.

Based on the above principle, one example of a device structure which iseffective in improving durability is shown below. In the devicestructure illustrated in FIG. 7, the emission layer is a mixed layerconsisting of the three kinds of a host material, a light emittingmaterial and another material. In order to achieve efficient luminancefrom the emitting material, it is preferable that (1) the energy bandgap of the emitting material is lower than both energy band gaps of thehost material and the another material; and (2) the ionization potentialof the host material is greater than the ionization potential of theemitting material.

In addition, to achieve the above-described suppression effect of holeaccumulation in the hole-transporting layer and the suppression effectof hole accumulation in the emitting material, the concentration of theanother material in the emission layer is preferably from 1% to 40%, andmore preferably from 5% to 30%. Meanwhile, the concentration of emittingmaterial is preferably from 0.1% to 40%, and more preferably from 1% to30%.

In terms of reliability, the host material is preferably a materialhaving good film-formability/durability, and usually a material that hasa high glass transition temperature. In addition, in order that theemitting material and the separate material are evenly dispersed in theemission layer, it is important to select a host material having goodcompatibility.

The emitting material may be fluorescent (luminescence from a singletexcited state) or phosphorescent (luminescence from a triplet excitedstate). However, when a phosphorescent emitting material is used, it isnecessary that the another material used in the emission layer has ahigher triplet energy than the phosphorescent emitting material in orderto achieve efficient luminance from the emitting material.

The organic light emitting device according to the present invention maybe constituted from a low molecular weight material, a high molecularweight material, or a combination thereof. Further, the emissionwavelength is not restricted, and may be from ultraviolet to visiblelight, or even infrared.

Although the material for emitting light may be a light emittingmaterial in which a plurality of emitting materials are mixed in theemission layer (e.g. a material which achieves white-light luminescencethrough mixing emitting materials of the colors red, green and blue inthe emission layer), in such a case, the ionization potential of theanother material must be smaller than the ionization potential of atleast one of the emitting materials, and preferably, the ionizationpotential of the another material has an ionization potential lower thanthe lowest ionization potential among the plurality of emittingmaterials.

Ionization potential and HOMO energy can be determined from UPS(ultraviolet photoelectron spectroscopy) or other method (e.g. measuringinstrument model AC-1 (manufactured by Riken Kiki Co., Ltd.), or bydetermining the oxidation potential by cyclic voltammetry. LUMO energycan be determined through calculation of the band gap value from lightabsorption and the HOMO energy or by determining the reduction potentialby cyclic voltammetry. Further, the HOMO and LUMO energy levels can bepredicted by simulated calculations using the molecular orbital method,density functional formalism or other methods.

Hole mobility can be measured by transient current measurement using aTOF (Time of Flight) method. It is suitable that the magnetic fieldstrength at the time of the measurement is about 1×10⁵ V/cm to about1×10⁶ V/cm.

Triplet energy can usually be determined from phosphorescence spectra.

Furthermore, known materials can be used as necessary in the organiclayers (the hole injection layer, hole-transporting layer,electron-transporting layer, emission layer etc.) which constitute thedevice.

Examples of these compounds will be given below.

A preferable hole-injecting/transporting material has excellent mobilityto facilitate the injection of a hole from an anode and to transport theimplanted hole to an emission layer. Low molecular and high molecularmaterials having hole-implanting/transporting abilities includetriarylamine derivatives, phenylene diamine derivatives, triazolederivatives, oxadiazole derivatives, imidazole derivatives, pyrazolinederivatives, pyrazolone derivatives, oxazole derivatives, fluorenonederivatives, hydrazone derivatives, stilbene derivatives, phthalocyaninederivatives, porphyrin derivatives, and poly(vinylcarbazole),poly(silylene), poly(thiophene), and other conductive polymers. However,the material is not limited to those compounds. Some specific examplesof the material will now be described.

Low-Molecular Weight Hole-Injecting/Transporting Materials

High-Molecular Weight Hole-Injecting/Transporting Materials

The electron-injecting/transporting material may be arbitrarily selectedfrom compounds which facilitate the injection of an electron from acathode and which has a function of transporting the implanted electronto an emission layer in consideration of the balance with the carriermobility of the hole-transporting material. Materials havingelectron-injecting/transporting abilities include oxadiazolederivatives, oxazole derivatives, thiazole derivatives, thiadiazolederivatives, pyrazine derivatives, triazole derivatives, triazinederivatives, perylene derivatives, quinoline derivatives, quinoxalinederivatives, fluorenone derivatives, anthrone derivatives,phenanthroline derivatives, and organometallic complexes. However, thematerial is not limited to those compounds. Some of the specificexamples of the compounds will be described below.

A fluorescent dye or phosphorescent material having high luminescenceefficiency can be used as the emitting material. Specific examplesthereof will be described below.

In the organic light emitting device according to the present invention,each layer containing an organic compound can be formed as a thin filmgenerally by a vacuum evaporation method, an ionization depositionmethod, a sputtering method, a plasma method, or a conventional coatingmethod (e.g., a spin coating, dipping, casting, LB, or inkjet method) inwhich the compound is dissolved in an appropriate solvent. In the caseof forming a film with a coating method, in particular, a film may beformed using the compound in combination with an appropriate binderresin.

Such a binder resin may be chosen from among a wide variety of resinshaving a binder property. Examples thereof include, but are not limitedto, polyvinylcarbazole resins, polycarbonate resins, polyester resins,polyarylate resins, polystyrene resins, ABS resins, polybutadine resins,polyurethane resins, acrylic resins, methacrylic resins, butyral resins,polyvinyl acetal resins, polyamide resins, polyimide resins,polyethylene resins, polyethersulfone resins, diallyl phthalate resins,phenol resins, epoxy resins, silicone resins, polysulfone resins, andurea resin. Each of those may also be used singly. Alternatively, two ormore of them may be mixed in combination as copolymers. Further,additives such as known plasticizers, antioxidants and ultravioletabsorbers may be used in combination if required.

An anode material preferably has as large a work function as possible.Examples of the anode materials include: metals such as gold, platinum,silver, copper, nickel, palladium, cobalt, selenium, vanadium, tungstenand chromium, and alloys thereof; and metal oxides such as tin oxide,zinc oxide, indium oxide, indium tin oxide (ITO), and indium zinc oxide.Further, conductive polymers such as polyaniline, polypyrrole,polythiophene, and polyphenylene sulfide may also be used. Each of thoseelectrode substances may be used singly. Alternatively, two or more ofthem may also be used in combination. Further, the anode may adopt anyone of a single layer construction and a multilayer construction.

On the other hand, a cathode material preferably has as small a workfunction as possible. Examples of the cathode material include: metalssuch as lithium, sodium, potassium, calcium, magnesium, aluminum,indium, ruthenium, titanium, manganese, yttrium, silver, lead, tin, andchromium; and alloys composed of multiple metals such as lithium-indium,sodium-potassium, magnesium-silver, aluminum-lithium,aluminum-magnesium, and magnesium-indium alloys. Metal oxides such asindium tin oxide (ITO) may also be used. Each of those electrodesubstances may be used singly. Alternatively, two or more of them mayalso be used in combination. Further, the cathode may adopt any one of asingle layer construction and a multilayer construction.

In addition, at least one of the anode and cathode is preferablytransparent or translucent.

A substrate which may be used in the present invention includes: opaquesubstrates such as a metal substrate and a ceramics substrate; andtransparent substrates such as of glass, quartz, and plastic sheet, butare not particularly limited to these materials. In addition, a colorfilter film, a fluorescent color converting film, a dielectricreflection film, or the like can be employed on the substrate to controlthe luminance color. Further, a device can be fabricated by forming athin-film transistor (TFT) on the substrate and connecting thereto.

Regarding the light-emitting direction of the device, either a bottomemission structure (light is emitted from the substrate side) or a topemission structure (light is emitted from the side opposite to thesubstrate) is possible.

Furthermore, a protective layer or a sealing layer may be formed on theprepared device to prevent the device from coming into contact withoxygen, moisture, or the like. The protective layer may be a diamondthin film, a film made of an inorganic material such as metal oxide ormetal nitride, a polymer film made of a fluorine resin, polyparaxylene,polyethylene, silicone resin, polystyrene resin, or the like, or may bea photo-curing resin, or the like. Furthermore, the device itself may becovered with glass, an airtight film, metal, or the like and packagedwith an appropriate sealing resin.

The present invention will be more specifically described with referenceto the following examples, but is not limited to the examples.

Example 1

An organic light emitting device having the structure shown in FIG. 7was prepared in the following manner.

A transparent conductive substrate was formed by sputtering indium tinoxide (ITO) on a glass substrate as a substrate to form thereon an anodewith a film thickness of 120 nm. Subsequently, this support substratewas subjected to ultrasonic cleaning in acetone and isopropyl alcohol(IPA) in order. Next, the support substrate was boiled and washed withIPA, followed by drying. Furthermore, the support substrate wassubjected to UV/ozone cleaning for use as a transparent conductivesubstrate.

Using the compound DFLDPBi represented by the following structuralformula as a hole-transporting material, a chloroform solution thereofwas prepared so that the concentration of the compound was 0.2% wt %.

By dropping this solution onto the above-described ITO electrode, it wasspin coated with the solution at a revolution speed of 500 rpm for 15seconds at first and then 1,000 rpm for 1 minute to form a thin filmthereon. After that, the resulting thin film was placed in a vacuum ovenand dried at 80° C. for 10 minutes to completely remove the solvent inthe thin film. The hole-transporting layer thus formed was 30 nm thick.Next, the following compound DPyFL as the host compound of the emissionlayer, the following luminescent compound s-DTAB2, and the followingcompound TFLFL were subjected to co-deposition (weight ratio=75:10:15)to form a 25 nm-thick emission layer 3. The degree of vacuum at the timeof deposition was 1.0×10⁴ Pa and the deposition rate was 0.1 to 0.2nm/second.

Further, using bathophenanthroline (Bphen), an electron-transportinglayer with a thickness of 30 nm was formed by a vacuum evaporationmethod. The degree of vacuum at the time of deposition was 1.0×10⁴ Paand the deposition rate was 0.1 to 0.2 nm/second.

Subsequently, a lithium fluoride film with a thickness of 0.5 nm wasformed on the above-described organic layer by a vacuum evaporationmethod, and a 150 nm-thick aluminum film was formed thereon by a vacuumevaporation method, to thereby prepare an organic light emitting device.For the lithium fluoride film, the degree of vacuum at the time ofdeposition was 1.0×10⁴ Pa and the deposition rate was 0.05 nm/second.For the aluminum film, the degree of vacuum at the time of depositionwas 1.0×10⁴ Pa and the deposition rate was 1.0 to 1.2 nm/second.

The resulting organic light emitting device was covered with aprotective glass plate and sealed with an acrylic resin-based binder ina dry air atmosphere to prevent the device from deteriorating due to theadsorption of moisture.

From the device thus obtained, the emission of s-DTAB2-derived greenlight was observed with an emission luminance of 6,000 cd/m² and amaximum emission wavelength of 530 nm at an applied voltage of 4.0 Vwhen the ITO electrode was provided as a positive electrode and the Alelectrode was provided as a negative electrode.

From an ionization potential determined using a measuring instrumentmodel AC-1 (manufactured by Riken Kiki Co., Ltd.), and a band gapobtained by measurement of ultraviolet-visible light absorption, theHOMO/LUMO energy levels of each material were investigated. Energydiagrams for the hole-transporting layer and the emission layer areshown in FIG. 8.

The ionization potential of TFLFL was 5.23 eV, which was smaller thanthat of the hole-transporting material DFLDPBi 5.42 eV, and also smallerthan that of the emitting material s-DTAB2 5.48 eV. Therefore, holeaccumulation in the hole-transporting layer and hole accumulation in theemitting material are expected to be moderated by TFLFL.

Furthermore, hole mobility was measured by transient current measurementusing the TOF (Time of Flight) method. Hole mobility for DFLDPBi was1×10⁻³ cm²/Vs, and that for TFLFL was 1×10⁻² cm²/Vs, which, inconjunction with their relative energy levels, strongly suggests thathole accumulation in the hole-transporting layer should be greatlymoderated.

When a voltage was applied to the device for 500 hours under a nitrogenatmosphere while current density was kept at 30 mA/cm², the rate ofluminance degradation after 500 hours was small because an initialluminance of about 5,700 cd/m² was changed to a luminance of about 4,000cd/m² after 500 hours.

Comparative Example 1

An organic light emitting device was prepared in the same manner as inExample 1, except that the emission layer was changed to a layer formedby co-deposition of DPyFL and s-DTAB2 (concentration ratio=90:10).

From the device thus obtained, the emission of s-DTAB2-derived greenlight was observed with an emission luminance of 3,500 cd/m² and amaximum emission wavelength of 530 nm at an applied voltage of 4.0 Vwhen the ITO electrode was provided as a positive electrode and the Alelectrode was provided as a negative electrode.

When a voltage was applied to the device for 500 hours under a nitrogenatmosphere while current density was kept at 30 mA/cm², luminancedegradation was greater than that of Example 1 because an initialluminance of about 4,800 cd/m² was changed to a luminance of about 2,100cd/m² after 500 hours. The difference was due to the absence of TFLFL,wherein it is thought that in Example 1, hole accumulation in thehole-transporting layer and hole accumulation in the emitting materials-DTAB2 was substantially moderated by TFLFL.

Comparative Example 2

An organic light emitting device was prepared in the same manner as inExample 1, except that the emission layer was changed to a layer formedby co-deposition of DPyFL, s-DTAB2 and αNPD (concentrationratio=75:10:15).

From the device thus obtained, the emission of s-DTAB2-derived greenlight was observed with an emission luminance of 3,600 cd/m² and amaximum emission wavelength of 530 nm at an applied voltage of 4.0 Vwhen the ITO electrode was provided as a positive electrode and the Alelectrode was provided as a negative electrode.

From the determination of ionization potential using a measuringinstrument model AC-1 (manufactured by Riken Kiki Co., Ltd.), theionization potential of the compound αNPD was 5.50 eV, which was greaterthan the ionization potential of the hole-transporting material DFLDPBibeing 5.42 eV and the ionization potential of the emitting materials-DTAB2 being 5.48 eV. This means that αNPD is not believed to functioneffectively as the above-described another material.

When a voltage was applied to the device for 500 hours under a nitrogenatmosphere while current density was kept at 30 mA/cm², luminancedegradation was greater than that of Example 1 because an initialluminance of about 4,700 cd/m² was changed to a luminance of about 2,100cd/m² after 500 hours. The difference is believed to be due to thedifference between TFLFL and αNPD, wherein αNPD has no effect ofmoderating hole accumulation in the hole-transporting layer andmoderating hole accumulation in the emitting material s-DTAB2.

Comparative Example 3

An organic light emitting device was prepared in the same manner as inExample 1, except that the emission layer was changed to a layer formedby co-deposition of DPyFL, s-DTAB2, and DFLDPBi (concentrationratio=75:10:15).

From the device thus obtained, the emission of s-DTAB2-derived greenlight was observed with an emission luminance of 3,700 cd/m² and amaximum emission wavelength of 530 nm at an applied voltage of 4.0 Vwhen the ITO electrode was provided as a positive electrode and the Alelectrode was provided as a negative electrode.

Since the ionization potential of the DFLDPBi co-deposited on theemission layer is the same as the ionization potential of thehole-transporting layer, (the hole-transporting layer also consisting ofDFLDPBi), DFLDPBi is not believed to function adequately as theabove-described another material.

When a voltage was applied to the device for 500 hours under a nitrogenatmosphere while current density was kept at 30 mA/cm², luminancedegradation was greater than that of Example 1, because an initialluminance of about 5,800 cd/m² was changed to a luminance of about 3,100cd/m² after 500 hours. The difference is believed to be due to thedifference between TFLFL and DFLDPBi, wherein DFLDPBi has little effectof moderating hole accumulation in the hole-transporting layer andmoderating hole accumulation in the emitting material s-DTAB2.

Example 2

An organic light emitting device was prepared in the same manner as inExample 1, except that the emission layer was changed to an emissionlayer having a total film thickness of 25 nm which consists of a 15nm-thick layer on the hole-transporting layer side which wasco-deposited with three kinds of DPyFL, s-DTAB2 and TFLFL (concentrationratio=75:10:15), and a 10 nm-thick layer which was subsequentlyco-deposited with two kinds of DPyFL and s-DTAB2 (concentrationratio=90:10).

From the device thus obtained, the emission of s-DTAB2-derived greenlight was observed with an emission luminance of 6,000 cd/m² and amaximum emission wavelength of 530 nm at an applied voltage of 4.0 Vwhen the ITO electrode was provided as a positive electrode and the Alelectrode was provided as a negative electrode.

When a voltage was applied to the device for 500 hours under a nitrogenatmosphere while current density was kept at 30 mA/cm², the rate ofluminance degradation after 500 hours was small because an initialluminance of about 5,700 cd/m² was changed to a luminance of about 3,900cd/m² after 500 hours.

Comparative Example 4

An organic light emitting device was prepared in the same manner as inExample 1, except that the emission layer was changed to an emissionlayer having a total film thickness of 25 nm which consists of a 15nm-thick layer on the hole-transporting layer side which wasco-deposited with two kinds of DPyFL and s-DTAB2 (concentrationratio=90:10), and a 10 nm-thick layer which was subsequentlyco-deposited with three kinds of DPyFL, s-DTAB2 and TFLFL (concentrationratio=75:10:15).

From the device thus obtained, the emission of s-DTAB2-derived greenlight was observed with an emission luminance of 3,500 cd/m² and amaximum emission wavelength of 530 nm at an applied voltage of 4.0 Vwhen the ITO electrode was provided as a positive electrode and the Alelectrode was provided as a negative electrode.

When a voltage was applied to the device for 500 hours under a nitrogenatmosphere while current density was kept at 30 mA/cm², luminancedegradation was greater than that of Example 1 because an initialluminance of about 4,800 cd/m² was changed to a luminance of about 2,300cd/m² after 500 hours. This is thought to be due to the fact thatbecause the hole-transporting side of the emission layer was not dopedwith TFLFL, hole injection from the hole-transporting layer to theemission layer did not proceed quickly.

Example 3

An organic light emitting device was prepared in the same manner as inExample 1, except that the electron-transporting material was changed to2,9-bis[2-(9,9-dimethylfluorenyl)]phenanthroline.

From the device thus obtained, the emission of s-DTAB2-derived greenlight was observed with an emission luminance of 6,000 cd/m² and amaximum emission wavelength of 530 nm at an applied voltage of 4.0 Vwhen the ITO electrode was provided as a positive electrode and the Alelectrode was provided as a negative electrode.

When a voltage was applied to the device for 500 hours under a nitrogenatmosphere while current density was kept at 30 mA/cm², the rate ofluminance degradation after 500 hours was extremely small because aninitial luminance of about 6,000 cd/m² was changed to a luminance ofabout 4,500 cd/m² after 500 hours.

Example 4

An organic light emitting device was prepared in the same manner as inExample 1, except that the emission layer was changed to a layer formedby co-deposition of DPyFL, st-BTAB2 and TFLFL (concentrationratio=75:10:15).

From the device thus obtained, the emission of st-BTAB2-derived greenlight was observed with an emission luminance of 6,000 cd/m² and amaximum emission wavelength of 525 nm at an applied voltage of 4.0 Vwhen the ITO electrode was provided as a positive electrode and the Alelectrode was provided as a negative electrode.

The measured ionization potential of the emitting material st-BTAB2 was5.49 eV.

When a voltage was applied to the device for 500 hours under a nitrogenatmosphere while current density was kept at 30 mA/cm², the rate ofluminance degradation after 500 hours was small because an initialluminance of about 5,500 cd/m² was changed to a luminance of about 3,900cd/m² after 500 hours.

Example 5

An organic light emitting device was prepared in the same manner as inExample 1, except that the emission layer was changed to a layer formedby co-deposition of t-DPyFL, BDT3FL and TFLFL (concentrationratio=80:10:10), and the electron-transporting material was changed to2,9-bis[2-(9,9-dimethylfluorenyl)]phenanthroline.

From the device thus obtained, the emission of st-BTAB2-derived bluelight was observed with an emission luminance of 1,000 cd/m² and amaximum emission wavelength of 470 nm at an applied voltage of 4.0 Vwhen the ITO electrode was provided as a positive electrode and the Alelectrode was provided as a negative electrode.

In addition, from ionization potential measurement using a measuringinstrument model AC-1 (manufactured by Riken Kiki Co., Ltd.), and a bandgap measurement with ultraviolet through visible light absorption, theHOMO/LUMO energy of each material was investigated. Energy diagrams forthe hole-transporting layer and the emission layer are shown in FIG. 9.

The ionization potential of the compound TFLFL was 5.23 eV, which wassmaller than the ionization potential of the hole-transporting materialDFLDPBi being 5.42 eV, and also smaller than the ionization potential ofthe emitting material BDT3FL being 5.31 eV. For this reason, the holeaccumulation in the hole-transporting layer and the hole accumulation inthe emission layer are expected to be moderated by TFLFL. Holeaccumulation in the emitting material BDT3FL is also expected to bemoderated.

When a voltage was applied to the device for 100 hours under a nitrogenatmosphere while current density was kept at 30 mA/cm², the rate ofluminance degradation after 100 hours was small because an initialluminance of about 1,200 cd/m² was changed to a luminance of about 830cd/m².

Comparative Example 5

An organic light emitting device was prepared in the same manner as inExample 5, except that the emission layer was changed to a layer formedby co-deposition of t-DPyFL and BDT3FL (concentration ratio=90:10).

From the device thus obtained, the emission of BDT3FL-derived blue lightwas observed with an emission luminance of 600 cd/m² and a maximumemission wavelength of 445 nm at an applied voltage of 4.0 V when theITO electrode was provided as a positive electrode and the Al electrodewas provided as a negative electrode.

When a voltage was applied to the device for 100 hours under a nitrogenatmosphere while current density was kept at 30 mA/cm², the rate ofluminance degradation was greater than that in Example 5 because aninitial luminance of about 1,000 cd/m² was changed to a luminance ofabout 600 cd/m² after 100 hours.

The difference was due to the absence of TFLFL, wherein it is thoughtthat hole accumulation in the hole-transporting layer and holeaccumulation in the emitting material BDT3FL are substantially moderatedby TFLFL.

This application claims priority from Japanese Patent Application No.2004-211231 filed Jul. 20, 2004, which is hereby incorporated byreference herein.

What is claimed is:
 1. An organic light emitting device comprising: ananode; a cathode; and a plurality of organic layers interposed betweenthe anode and the cathode, wherein the plurality of organic layersincludes a first layer which is an emission layer, a second layer whichis in contact with the emission layer on an anode-side of the emissionlayer, and a third layer which is in contact with the emission layer ona cathode-side of the emission layer, wherein the first layer includes ahost material, a light emitting material, and another material, whereinthe host material and the light emitting material have an ionizationpotential larger than an ionization potential of a material of thesecond layer, wherein the LUMO of the another material is smaller thanthe LUMO of a material of the third layer, wherein the light emittingmaterial has a band gap smaller than both band gaps of the host materialand the another material, wherein the LUMO of the another material issmaller than the LUMO of the host material, and the LUMO of the hostmaterial is smaller than the LUMO of the material of the third layer,wherein the first layer includes a different light emitting materialfrom the light emitting material, wherein the another material has anionization potential smaller than an ionization potential of at leastone of the light emitting material and the different light emittingmaterial from the light emitting material, and wherein the organic lightemitting device emits a white color.
 2. An apparatus comprising asubstrate, the organic light emitting device according to claim 1 and acolor filter, wherein the substrate has the organic light emittingdevice thereon.
 3. An apparatus comprising a substrate and the organiclight emitting device according to claim 1, wherein the substrate hasthe organic light emitting device thereon, and the apparatus has a topemission structure.